We are building a second floor bathroom over a first floor bathroom. We are draining (and venting) the fixtures into the vent for first floor fixtures (creating a wet stack, which I understand may not be to code, but will function properly?).
Question: we will also have an exhaust fan over the shower, which needs to be vented. Builder wants to tap that vent into either (1) the existing wet vent previously described, or (2) the radon vent pipe that passes through that space. Will either of these work? Will tapping into radon pipe create dangerous condition?
The alternative seems to be to cut a third hole in the roof, but that is more work and creates another area for a potential leak.
Thanks!

I would not even consider the radon pipe and I'm certain the inspector will have an issue with it. Code is fairly clear and doesn't include any language relating to the sharing of another vent pipe. Your safety is considered when the code was written... another roof vent does not imply another leak.

Your builder wants to tap your bathroom ventilating system into the sewer or into a radiation exhaust vent? He must hate you and your family very much to suggest such a thing. What did you ever do to the builder?

You say,"We are draining (and venting) the fixtures into the vent for first floor fixtures (creating a wet stack, which I understand may not be to code, but will function properly?"

No it will not function properly. What you have done is to change the first floor vent to a second floor main drain and then discharge major fixtures past a unvented bathroom group. By converting the first floor vent to a second floor drain you also converted every "P" trap in the first floor bathroom to a "S" trap. "S" traps have been outlawed for years in both that States and Canada. ( This just keeps getting better and better, don't it?! ) There's a reason for vents and your builder is running rough shod over them. Does he not have a plumber to advise him or is this the type of work he does on a regular basis? I can't believe a licensed contractor would even suggect so many code violations . And the plumber that installed the upstairs bathrom group. There couldn't have been a inspection or your house would have been red tagged at once. Sounds to me you hired Cheech and Chong to do your work.
However all is not lost. This can still be saved. First exhaust the bathroom fan out the roof or into the attic. And next, disconnect the first floor lavatory vent from the second floor main drain and run a 2" pipe up next to the drain past the second floor bathroom and revent it back into the original vent at least 6" over the flood rim of the highest fixture. If the builder or the plumber doesn't under stand these technical terms I'l be more then happy to dumb it down for them. My friend, do not allow anybody to trash your house out and then charge you for it. Call your building department, ask for code violatations and have them make the builder bring your house up to code at no charge to you. You can save this post and show it to the inspector they send out. Good luck and please let me know the outcom. When I see work like this it always makes my blood boil. We plumbers don't make much of it but we hold the health of the nation in our hands. A wrong connection, a crossed line and a family is open to all kinds on nasty diseases. A good plumber knows and respects that. And then again there are some that don't.
Regards and keep me informed, Tom

1. We are exhausting the new fan direct out the roof.
2. will fix venting/drain issue with fixtures and first floor bathroom.
Thanks for your input and guidance on solving problem!
Steve :)